High sulfur iron has been treated external to a melt unit, typically in the ladle or launder, by employing suitable agents consisting of sodium carbonate, calcium carbonate,burnt lime, calcium cyanamide, calcium carbide and rare-earths. Of all these materials, the desulfurizing potential of calcium carbide, using the porous plug and/or injection of calcium carbide with nitrogen gas, is universally well recognized. The reaction kinetics of sulfur removal from the metal largely depend upon how intimately the metal and the desulfurizing agent is mixed and this is achieved in actual practice by blowing nitrogen gas through a porous refractory at the bottom of the ladle. Calcium sulfide and unreacted calcium carbide and oxides that are formed go into the slag which is removed from the metal. Due to the potential hazards of forming acetylene gas when contacted with moisture, the calcium carbide and the resultant slag requires very careful handling.
Little recognition has been given to the desulfurizing effects of magnesium and calcium metals. Magnesium has been principally appreciated as a nodularizing agent for iron and has required the use of a relatively low sulfur iron. Laboratory investigations have shown that where the sulfur is more than 0.01%, which is the maximum desired for nodularization, the magnesium will first desulfurize and then nodularize. In spite of the fact that magnesium can achieve desulfurization and nodularization simultaneously, it is accepted that it is most difficult to retain magnesium in the molten iron so that it can desulfurize. This difficulty arises from a variety of physical characteristics which include: (a) the typical treatment temperature for molten iron is usually at about 2600-2800.degree. F. and magnesium is in vapor form at that temperature level; (b) the solubility of magnesium into molten iron is extremely low; (c) magnesium oxidizes extremely rapidly when it comes into contact with air; (d) magnesium is a very light material and due to its low density tends to float on the molten metal and become oxidized; and (e) magnesium is extremely reactive with molten iron and produces considerable pyrotechnic display which may consist of bursts of iron particles due to such reactivity.
The prior art has attempted to carry out the magnesium reaction according to principally four methods: the sandwich method, the injection method, the plunging process, and the Fisher or Kuboto processes requiring a pressure tight reaction chamber. The sandwich method involves diluting the magnesium with nickel or silicon, etc., such as in an alloy form, so that when the diluted material is brought into contact with the molten iron, which is preferably laid on the bottom of the molten vessel. A reduced magnesium vapor pressure will result and thus retard the tendency to send off magnesium vapors with extreme reactivity. Examples of magnesium alloys include Mg-Ni and Mg-Fe-Si. Unfortunately the latter alloy is insufficiently heavy, as compared to the Ni-Mg alloy, so that additional steel cover of particles is necessary to prevent it from floating in the reaction ladle. The difficulty with the sandwich method is that the recovery of magnesium is only about 30-50% of the magnesium that is added to the process. Recovery shall mean herein the ratio between the units of a material added to a process and the units of the material appearing in the final metal product plus that combined with impurities.
Although not commercially used, the injection of magnesium powder takes place by use of an inert vehicle, such as nitrogen gas. It is typical for such magnesium powder to carry an oxide coating thereon by the mere nature of the production of the magnesium particles. Therefore, the recovery of pure magnesium in the final metal as compared to that which is utilized at the starting point, is extremely low (30% recoveries are typical).
The plunging process uses a block of pure magnesium coated with layers of a suitable refractory; or a coke body impregnated with pure or high magnesium, each of which are plunged into the molten bath of iron. If carried out in a conventional way with the plunging tool introduced from the top of an open ladle and carried close to the bottom of the vessel, the recovery of magnesium again is typically about 30-50%.
The Kuboto process includes the introduction of pure magnesium to a body of molten iron in a sealed pressure vessel. The vessel must withstand pressures of at least 15 atmospheres, the vapor pressure that will be immediately produced by the magnesium iron reaction. The iron is supersaturated with magnesium and then diluted with the unreacted molten metal to achieve the desired magnesium content. The Fisher process again includes introduction of pure magnesium to a body of molten iron through a reaction chamber to a loosely closed vessel. Magnesium is retained under the molten iron in a small pocket containing holes through which the reaction proceeds until completed. Unfortunately, the recovery of the magnesium by either Kuboto or Fisher is still relatively low, at around the 40% level.